Alpha-alumina is widely used as an abrasive material in the abrasives industry. It may be used in a pure form, or more preferably in a form containing additives that enhance its abrasive properties. Many commercially important alpha-alumina abrasive particles are derived from a sol-gel precursor. They are made by preparing a dispersion (e.g., a sol) comprising water, an alpha-alumina precursor such as, e.g., alumina monohydrate (boehmite), and optionally peptizing agent (e.g., an acid such as nitric acid), then gelling the dispersion, drying the gelled dispersion, crushing the dried dispersion into particles, calcining the particles to remove volatiles, and sintering the calcined particles at a temperature below the melting point of alpha-alumina. Frequently, the dispersion also includes one or more oxide modifiers (e.g., rare earth oxides (REOs), Cr2O3, CoO, Fe2O3, Li2O, MgO, MnO, Na2O, NiO, SiO2, SnO2, TiO2, ZnO, and ZrO2), seed particles (e.g., alpha-alumina seed particles and alpha-Fe2O3 seed particles) and/or precursors thereof. Such additions are typically made to alter or otherwise modify the physical properties and/or microstructure of the sintered abrasive particles. In addition, or alternatively, such oxide modifiers, nucleating agents, and/or precursors thereof may be impregnated into the dried or calcined material (typically calcined particles). Among the most useful of the oxide modifiers are REOs such as, for example, lanthanum oxide, neodymium oxide, yttrium oxide, cerium oxide, europium oxide, hafnium oxide, erbium oxide, samarium oxide, ytterbium oxide, gadolinium oxide, and praseodymium oxide. For example, REO modifiers are commonly used in sol-gel alumina abrasives for use in abrading stainless steel. In recent years, the availability of REOs has become a major problem for all industries, limiting supply and driving up costs.
Sol-gel-derived alpha-alumina-based sintered abrasive particles have been used in a wide variety of abrasive products (e.g., bonded abrasives, coated abrasives, and abrasive brushes) and abrading applications, including both low and high pressure grinding applications.